where to run skimmer outlet?

[cmbspd]

I'm setting up my first reef tank using a 100 gallon stock tank in the basement. I plan on placing a second rubbermaid container inside for the drain and protein skimmer. I've checked out a lot pics for these types of setups and I've got a question. Why do they all seem to have the outlet for the protein skimmer simply flowing back into the internal container rather than routing to the main sump. It seems like this ends up "reskimming" the clean water.

Is it that you cannot route the skimmer effluent into the main sump tank and still maintain a constant water level in the skimmer chamber?

Thanks!